Quantum Computing (QUBT) had the highest return in January 2018 by a mid-cap US stock or mid-cap US stock, returning 650%.

ASSET	MONTH	% RETURN
Quantum Computing (QUBT)	January 2018	650%
Skyline (SKY)	January 2018	72.98%
Arrowhead Pharmaceuticals (ARWR)	January 2018	67.48%
Crispr Therapeutics (CRSP)	January 2018	63.67%
Perpetua Resources (PPTA)	January 2018	62.2%
PTC Therapeutics (PTCT)	January 2018	55.93%
Applied Digital (APLD)	January 2018	48.33%
Denali Therapeutics (DNLI)	January 2018	45.38%
American Superconductor (AMSC)	January 2018	40.81%
TG Therapeutics (TGTX)	January 2018	38.55%
Kenon (KEN)	January 2018	37.81%
Agios Pharm (AGIO)	January 2018	36.38%
Intellia Therapeutics (NTLA)	January 2018	32.82%
Companhia Siderurgica Nacional ADR (SID)	January 2018	32.81%
Liberty Oilfield Services (LBRT)	January 2018	30.24%
Viking Therapeutics (VKTX)	January 2018	29.83%
Surgery Partners (SGRY)	January 2018	28.51%
Zai Lab (ZLAB)	January 2018	27.77%
Corcept Therapeutics (CORT)	January 2018	27.37%
Autohome (ATHM)	January 2018	26.76%
Galapagos NV ADR (GLPG)	January 2018	25.85%
New York Times (NYT)	January 2018	25.27%
OneMain (OMF)	January 2018	24.99%
GDS (GDS)	January 2018	24.3%
Weibo (WB)	January 2018	23.86%

Mid-cap stocks are treated as having a market cap between $2B and $10B.